It has been recognized that due to environmental concerns and newly enacted rules and regulations, saleable petroleum products must meet lower and lower limits on contaminates, such as sulfur. New regulations require essentially complete removal of sulfur from liquid hydrocarbons that are used in transportation fuels, such as gasoline and diesel. For example, ultra low sulfur diesel (ULSD) requirements are typically less than about 10 ppm sulfur.
Fluid catalytic cracking (FCC) is a unit operation used by the petroleum industry to convert higher boiling hydrocarbons to more valuable products, such as gasoline. FCC feedstocks also have contaminants that require removal to meet environmental emission regulations and low sulfur product quality requirements for the FCC operation to be efficient and economical. For example, high levels of nitrogen and lower hydrogen content of the FCC feed is generally undesirable. High levels of nitrogen have undesired effects on the cracking catalysts, and lower hydrogen content results in higher severity operation and high coke formation in the FCC process. Hydrotreating or mild hydrocracking of the FCC feed allows the reduction of contaminants and increases the hydrogen content of the FCC feed. Typically mild hydrocracking may be used to pretreat the FCC feed and produce limited amounts of diesel. Unfortunately, the lower conversions in the mild hydrocracking usually results in lower quality diesel product. On the other hand, higher pressure operation is typically more expensive and usually results in overtreatment of the FCC feed.
Such dual requirements, as the desire to produce high quality diesel while not overtreating the FCC feed and efficient and economic use of hydrogen, often present a dilemma in attempting to set proper severity of operation. On one hand, there is interest in the use of lower conversion and lower pressure conditions to produce FCC quality feedstocks while, on the other hand, higher quality diesel production requires higher conversion and higher pressure. Mild hydrocracking conditions using moderate temperatures and pressures can produce FCC quality feedstocks having acceptable levels of sulfur and nitrogen. Such conditions typically may be used in less costly hydrocracking reactors that do not need to withstand high temperatures and pressures typical of more severe conditions. The mild hydrocracking conditions generally at lower conversion produce a lower quality product that generally does not meet the stringent ULSD requirements, but produces acceptable FCC feedstocks.
On the other hand, to achieve high quality product that meets ULSD levels, higher conversion rates and higher pressures can be employed. However, to remove sufficient quantities of sulfur and produce high quality product to meet ULSD levels, relatively high hydrogen pressures are generally required. Not only do such high hydrogen pressures require extensive capital investment in recycle gas compressors and other equipment, the high hydrogen pressures also result in FCC feedstocks that are overtreated with excessive hydrogen. Overtreated FCC feed is uneconomical to process because it can result in over cracking in the FCC unit producing lighter, lower boiling point products, rather than the desired gasoline or other desired hydrocarbon-based products.
Although a wide variety of process flow schemes, operating conditions and catalysts have been used in commercial petroleum hydrocarbon conversion processes, there is always a demand for new methods and flow schemes that provide economical operations to produce more useful products and improved product characteristics. In many cases, even minor variations in process flows or operating conditions can have significant effects on both quality and product selection. There generally is a need to balance economic considerations, such as capital expenditures and operational utility costs, with the desired quality of the produced products.
There is a continuing need, therefore, for improved and cost effective methods to produce hydrocarbon streams that meet increasingly stringent products requirements. In particular, there is a need to provide both FCC quality feedstocks and high quality (cetane) ULSD from hydrocarbonaceous streams in a cost effective and efficient manner.